1. Field of the Invention
The present invention relates to an optical position detecting method, wherein an image of a light source as an object to be measured is captured by an image sensor so as to measure a position of the light source. More specifically, the present invention relates to a technique for removing noise which is caused by ambient sunlight, illumination light and the like when performing such an optical measurement.
2. Description of the Prior Art
Optical position detecting methods have been known in the art, wherein an image of a light source as an object to be measured is captured by an image sensor so as to measure a position of the light source, for example as disclosed in Japanese First (unexamined) Patent Publication No. 6-59807. In this publication, a position control of a cursor displayed on the display of a computer is performed using a light pointer. Specifically, a point light source incorporated in the light pointer is captured by the image sensor incorporated in the computer body so as to extract a position of the point light source by performing the image processing. Based on the extraction result, the cursor position is controlled. The disclosed optical position detecting method has also been utilized in a survey in the civil engineering field and so on, other than the light pointer and the like which are operated inside a room. Specifically, the survey is performed by setting a position indicator including therein a point light source, such as, LED, at a measurement site and by capturing an image of the position indicator using a television camera or the like at a distant place.
In the position measurement using a light beam as a measuring medium, it is necessary to remove noise factors caused by ambient light, such as, the sunlight and the interior illumination light. Particularly, when a distance between the light source and the image sensor is increased, intensity of the light emitted from the light source is weakened, and thus, intensity of the ambient light is relatively increased so that the precise position measurement is impeded. In view of this, various countermeasures have been taken for removing noise. For example, by subtracting from an active image data obtained by capturing the light source in a light-on state a passive image data obtained by capturing the light source in a light-off state, a net signal component is extracted so as to eliminate the influence of the ambient light. However, in such a conventional method, since it is necessary to synchronize light-on and light-off timings at a side of the light source with image capturing timings at a side of the image sensor, the structure inevitably becomes complicated. For example, a cable becomes necessary for synchronizing the foregoing timings, which further makes it difficult to realize a wireless structure at the remote site of the light source. In case of the light pointer, for example, since the wireless light pointer is difficult to realize, the wired light pointer seriously spoils maneuverability of the light pointer.